Delayed onset muscle soreness (DOMS) describes a phenomenon of muscle pain, muscle soreness or muscle stiffness that is felt 12-48 hours after exercise, particularly at the beginning of a new an exercise program, after a change in sports activities, or after a dramatic increase in the duration or intensity of exercise.
The delayed soreness of DOMS is generally at its worst within the first 2 days following the activity and subsides over the next few days.

Causes:
Delayed onset muscle soreness is thought to be a result of microscopic tearing of the muscle fibers. The amount of tearing (and soreness) depends on how hard and how long you exercise and what type of exercise you do.
eccentric muscle contractions, movements that cause muscle to forcefully contract while it lengthens seem to cause the most soreness.
Examples of eccentric muscle contractions include going down stairs, running downhill, lowering weights and the downward motion of squats and push-ups. In addition to small muscle tears there can be associated swelling in a muscle which may contribute to soreness.

Treatment:
There is no one simple way to treat delayed onset muscle soreness. In fact, there has been an ongoing debate about both the cause and treatment of DOMS. In the past, gentle stretching was one of the recommended ways to reduce exercise related muscle soreness, but a study by Australian researchers published in 2007 found that stretching is not effective in avoiding muscle soreness.
Nothing is proven effective, it’s best for an individual to try a few things to see what works for them.

Dealing with soreness after exercise:
Wait. Soreness will go away in 3 to 7 days with no special treatment.
Try an ice bath or contrast water bath. Although no clear evidence proves they are effective, many pro athletes use them and claim they work to reduce soreness.
Active Recovery (low intensity exercise after intense workouts)This strategy does have some support in the research. Perform some easy low-impact aerobic exercise to increase blood flow. This may help diminish muscle soreness.
Gently massage the affected muscles.
Try using a non steroidal anti-imflammatory medication such as aspirin or ibuprofen to reduce the soreness temporarily, though they won’t actually speed healing.
Allow the soreness to subside thoroughly before performing any vigorous exercise.
Certain muscle pain or soreness can be a sign of serious injury. If your muscle soreness does not get better within a week consult your physician.
Don’t let muscle soreness after starting a new fitness routine scare you from returning to activity. It won’t be as bad as it was the first time around, as long as you stick to it!